Defining the rate at which T cells turn over has important implications for our understanding of T lymphocyte homeostasis and AIDS pathogenesis, yet little information on T cell turnover is available. We employed the nucleoside analogue bromodeoxyuridine (BrdU) in combination with five color flow cytometric analysis to evaluate T lymphocyte turnover rates in normal and simian immunodeficiency virus (SIV)-infected rhesus macaques. T cells in normal animals turned over at relatively rapid rates, with memory cells turning over more quickly than naive cells. In SIV-infected animals, the labeling and elimination rates of both CD4+ and CD8+ BrdU-labeled cells were increased by 2 to 3-fold as compared with normal controls. In normal and SIV-infected animals, the rates of CD4+ T cell BrdU-labeling and decay were closely correlated with those of CD8+ T cells. The elimination rate of BrdU-labeled cells was accelerated in both naive and memory T lymphocytes in SIV-infected animals. Our results provide direct evidence for increased rates of both CD4+ and CD8+ T cell turnover in AIDS virus infection and have important implications for our understanding of T cell homeostasis and the mechanisms responsible for CD4+ T cell depletion in AIDS.